This invention relates generally to optical communications. More particularly, the invention relates to switches for fiber optic communications systems.
Silicon-micromachining technology has been used to fabricate micro-optical devices such as movable micromirrors in order to build miniaturized optical components and communications subsystems. Using silicon-micromachining and silicon optical bench technologies, extremely compact optical components and systems incorporating fiber optic technologies can be built for communication and test-and-measurement instrumentation applications.
Microelectromechanical systems (MEMS) are miniature mechanical devices manufactured using the techniques developed by the semiconductor industry for integrated circuit fabrication. Such techniques generally involve depositing layers of material that form the device, selectively etching features in the layer to shape the device and removing certain layers (known as sacrificial layers, to release the device. Such techniques have been used, for example, to fabricate miniature electric motors as described in U.S. Pat. No. 5,043,043.
Recently, MEMS devices have been developed for optical switching. Such systems typically include an array of mechanically actuatable mirrors that deflect light from one optical fiber to another. The mirrors are configured to translate or rotate into the path of the light from the fiber. Mirrors that rotate into the light path generally rotate about a substantially horizontal axis, i.e., they xe2x80x9cflip upxe2x80x9d from a horizontal position into a vertical position. MEMS mirrors may be actuated by magnetic interaction, electrostatic interaction, or some combination of both.
Modern fiber optic communication networks utilize fiber cable trunk lines containing a plurality of fiber optic strands for routing the traffic in the fibers to designated destinations. The number of fibers or optical ports continues to grow as the traffic load increases. Consequently multiple ( greater than 2) port fiberoptic components and subsystems are becoming more popular. Of particular interest are optical add/drop multiplexers (OADMs) that can be constructed using a micromirror array in conjunction with fiber arrays for optical inputs and outputs. As the number of add/drop/pass channels increases, the arrangement of the fibers in the array becomes critical as a proportional scaling of component size with channel count does not make an ideal solution for all applications.
A typical component of an optical communications system is an optical crossbar switch. Optical crossbar switches that utilize a single movable mirror are described, for example, in U.S. Pat. No. 4,932,745. FIG. 1A shows a typical 2xc3x972 crossbar switch 100 of the prior art. The switch 100 optically couples light signals between inputs IN 1, and outputs Out 1, Out 2. The switch 100 generally comprises a mirror 102 movable disposed on a substrate 104. The mirror 102 includes reflecting surfaces on a front side 106 and a back side 108. The mirror 102 translates horizontally between a first position and a second position. In the first position the mirror 102 blocks direct optical paths between inputs IN 1, IN 2 and outputs Out 2, Out 1 respectively. Light from IN 1 reflects off the back surface 108 to Out 1. Light from IN 2 reflects off the front surface 106 to Out 2. In the second position, mirror 102 is removed from the direct path of light between the inputs and the outputs such that light from IN 1 travels directly to Out 2 and light from IN 2 travels directly to Out 1.
Switch 100 requires very precise alignment between the inputs and the outputs. Because the inputs are not parallel to each other, alignment of the inputs with the outputs is difficult. Furthermore, it is difficult, if not impossible to scale up the basic 2xc3x972 crossbar switch to larger numbers of inputs and outputs. Furthermore, the input and output fibers have to be assembled individually, which is not conducive to the construction of large port-count components.
An alternative crossbar switch is described in U.S. Pat. No. 5,841917. FIGS. 2A-2B depict a crossbar switch 200 that is compatible with parallel arrays of inputs and outputs. The switch 200 generally comprises a 2xc3x972 array of movable mirrors 2021, 2022, 2023, and 2024. The movable mirrors selectively couple two parallel inputs IN 1 and IN 2 to two parallel outputs Out 1 and Out 2 that are oriented at right angles to IN 1 and IN 2. Each mirror is oriented at an angle of approximately 45xc2x0 with respect to the orientation of both the inputs and the outputs. Consequently mirrors 202 deflect the path of light beams from the inputs by 90xc2x0 to direct them towards the outputs. As in the crossbar switch 100 of FIG. 1, two switching states are possible. For example, in FIG. 2A mirrors 2021 and 2024 are in an xe2x80x9cupxe2x80x9d position and mirrors 2022 and 2023 are in a xe2x80x9cdownxe2x80x9d position. Mirror 2021 deflects light from input IN 1 toward output Out 1 while mirror 2024 deflects light from input IN 2 toward output Out 2. Alternatively, in FIG. 2B, mirrors 2021 and 2024 are in the xe2x80x9cdownxe2x80x9d position and mirrors 2022 and 2023 are in the xe2x80x9cupxe2x80x9d position. Mirror 2022 deflects light from input IN 1 toward output Out 2 while mirror 2023 deflects light from input IN 2 toward output Out 1.
Because the inputs IN 1, IN 2 are parallel to each other and outputs Out 1, Out 2 in switch 200, alignment is greatly simplified. Furthermore, switch 200 may be readily scaled up to accommodate any number of fibers M in an MXM array, where M is an integer greater than 2. However, because the input fiber array and the output fiber array are oriented at an angle with respect to each other, alignment of the inputs and outputs with the mirrors is problematic. Furthermore, as the number of fibers M becomes large, the number of mirrors and the area they occupy, scales as M2. Thus, for very large-scale arrays the switch occupies a large amount of space, which can be a serious disadvantage when the space available for the switch is limited.
There is a need, therefore, for an optical switching apparatus that is easier to align, uses fewer mirrors, and occupies less space.
Accordingly, it is a primary object of the present invention to provide a layout for micromachined mirrors that facilitates the optimal arrangement of the mirrors. It is a further object of the invention to provide a layout that facilitates alignment of the input and output ports. It is an additional object of the invention to provide a layout for an optical switching module that facilitates miniaturization the dimensions of the resulting components or subsystems.
These objects and advantages are achieved by the present invention of an optical switch module, comprising at least two fixed mirrors and at least one movable mirror. In a first embodiment, the movable mirror is typically disposed between the two fixed mirrors with all three mirrors aligned parallel to each other in a linear array to form a crossbar switch. The mirrors couple optical signals between two or more inputs and two or more outputs. The movable mirror is movable between a first position and a second position. In the first position, a first fixed mirror and the movable mirror deflect light from a first input to a first output and light from a second input to a second output. The movable mirror and a second fixed mirror deflect light from a second input to a second output. In the second position, the first and second fixed mirrors deflect light from the first input to the second output and light from the second input travels straight to the second output.
In a second embodiment, the basic switch module may be scaled up to form an apparatus that incorporates N movable mirrors and N+1 fixed mirrors, where N is an integer greater than zero. Such an apparatus can accommodate 2N fiber inputs and 2N fiber outputs. According to a third embodiment of the invention, the apparatus implements an optical add/drop multiplexer (OADM). The multiplexer includes a number of add ports coupled to selected inputs and a number of drop ports coupled to selected outputs. The add ports allow a local device to receive information from a main input fiber. The drop ports allow a local device to send information to a main output fiber.
In a third embodiment of the invention a crossbar switch is fabricated with inputs and outputs arranged in collimated fiber arrays. A parallel configuration of the fibers and the collimators facilitate fabrication of the apparatus and alignment of the input fibers. The parallel configuration of the switch module takes advantage of existing lens array and fiber V-groove technology to facilitate integration of the fibers and collimators in the module, thereby reducing the difficulty and cost associated with alignment, assembly, and packaging.